This image shows a time sequence
of Hubble Space Telescope images of the light
echo around V838 Mon, taken between May 2002 and
October 2004. All six pictures were taken with
Hubble's Advanced Camera for Surveys using filters
sensitive to blue, visible, and infrared wavelengths.
The apparent expansion of the light echo, as light
from the early 2002 outburst of V838 Mon propagates
outward into the surrounding dust, is clearly
shown.

All of the images are shown at the
same scale. Moreover, the images are also shown
as they would appear for the same exposure times
throughout the sequence. Thus the background stars
appear constant in brightness, while the surface
brightness of the light echo steadily declines.
The fading of the light echo is primarily due
to the light-scattering properties of interstellar
dust. Consider a street lamp on a foggy night.
The halo around the lamp is brightest right next
to the lamp, while out to the side it is much
fainter. Similarly, in the first V838 Mon image,
taken in May 2002, the light echo was very bright
and compact. At later times, we are seeing dust
out to the side of the star, rather than dust
that is immediately in front of the star, so the
amount of light scattered in our direction is
smaller. Hubble astronomers expect the light echo
to continue to change its appearance and brightness
over the next several years.

A dissolve sequence of six images
taken by Hubble's Advanced Camera for Surveys
shows a CAT-scan-like probe of the three-dimensional
structure of the shells of dust surrounding the
aging star V838 Monocerotis. The sequence reveals
dramatic changes in the way a brilliant flash
of light from the star is reflecting off surrounding
dusty cloud structures. The effect, called a light
echo, has been unveiling never-before-seen dust
patterns ever since the star suddenly brightened
for several weeks in early 2002.
TRT = 0:30

Animation Credit: NASA, ESA, and
G. Bacon (STScI)

Questions and Answers

LIGHT CONTINUES TO ECHO
THREE YEARS AFTER STELLAR OUTBURST

The Hubble Space Telescope's latest
image of the star V838 Monocerotis (V838 Mon)
reveals dramatic changes in the illumination of
surrounding dusty cloud structures. The effect,
called a light echo, has been unveiling never-before-seen
dust patterns ever since the star suddenly brightened
for several weeks in early 2002.

1. What is a light echo?

It is light from a stellar explosion
echoing off dust surrounding the star. V838 Monocerotis
produced enough energy in a brief flash to illuminate
surrounding dust, like a spelunker taking a flash
picture of the walls of an undiscovered cavern.
The star presumably ejected the illuminated dust
shells in previous outbursts. Light from the latest
outburst travels to the dust and then is reflected
to Earth. Because of this indirect path, the light
arrives at Earth months after light from the star
that traveled directly toward Earth.

2. Why did the star produce
this outburst?

Astronomers do not fully understand
the star's outburst. It was somewhat similar to
that of a nova, a more common stellar outburst.
A typical nova is a normal star that dumps hydrogen
onto a compact white- dwarf companion star. The
hydrogen piles up until it spontaneously explodes
by nuclear fusion -- like a titanic hydrogen bomb.
This exposes a searing stellar core, which has
a temperature of hundreds of thousands of degrees
Fahrenheit.

By contrast, V838 Monocerotis did
not expel its outer layers. Instead, it grew enormously
in size. Its surface temperature dropped to temperatures
that were not much hotter than a light bulb. This
behavior of ballooning to an immense size, but
not losing its outer layers, is very unusual and
completely unlike an ordinary nova explosion.

The outburst may represent a transitory
stage in a star's evolution that is rarely seen.
The star has some similarities to highly unstable
aging stars called eruptive variables, which suddenly
and unpredictably increase in brightness.

3. Are light echos similar
to sound echos?

The echoing of light through space
is similar to the echoing of sound through air.
As light from the stellar explosion continues
to propagate outwards, different parts of the
surrounding dust are illuminated, just as a sound
echo bounces off of objects near the source, and
later, objects further from the source. Eventually,
when light from the back side of the nebula begins
to arrive, the light echo will give the illusion
of contracting, and finally it will disappear.

4. Where is V838 Mon located?

V838 Mon is located about 20,000
light-years away from Earth in the direction of
the constellation Monoceros, placing the star
at the outer edge of our Milky Way galaxy.

5. How can the visible structure
around the star grow from 4 to 7 light years in
less than a year's time?

One of the properties of light echoes
is that they appear to violate the limit imposed
by the speed of light, but this in only a geometric
illusion. The echo does indeed appear to expand
at greater than the speed of light, due to the
peculiar geometry of a light echo. The dust illuminated
at the outer edge of the echo is indeed about
5 light years from the star, but this dust is
WELL IN FRONT of the star, not at the same distance
as the star. So, this light has only gone slightly
out of its way off to the side, and was then reflected
in the direction toward the Earth. The light did
not travel faster than the noted speed of light.

This illusion that the speed of
light has been violated can be understood in more
detail if you take a look at the graphic called
Anatomy of a Light
Echo, and consider one of the paraboloids,
such as the one labeled "2". At one
particular time, all of the dust along this paraboloid
is illuminated, all the way down to the end of
the parabola at the lower right. This illuminated
material down at the end appears to lie at the
largest distance from the star, as seen from Earth,
but as you can see it is well in front of the
star and only slightly off to the side.

Now follow the parabola up to the
vicinity of the star, and consider in particular
the point where the angle from the star and then
to the Earth is 90 degrees. The distance from
the star to THAT point is how far the light has
traveled, at the speed of light, in the plane
of the sky, which as you can see is much less
than the apparent radius from the star that has
been reached down at the lower right, as seen
from Earth.